Ion generation devices (also called ion generators, ion sources, ion guns, etc.) are in widespread use in the industrial field for surface treatments (ion etching, cleaning, deposition of materials, ion implantation, etc.) and chemical and physical analysis (for example, the determination of the type and orientation of crystals on the surface of a solid). In the space field such devices are used as ion engines and, on earth, for the generation of simulated ionospheric plasma.
A known device for the generation of ions is schematically shown in FIG. 1. This comprises an ionization chamber 1 and an extraction system 2. A substance in the gas or vapor state, from which the positive ions of the desired chemical type are obtained by various techniques known per se, is introduced into the ionization chamber. Such ions are then extracted from the ionization chamber, focused, and accelerated toward the lens of the extraction system 2. Other parts present in the device will not be mentioned here since they are not relevant to the description of the present invention. A plasma is generated in the ionization chamber, and contains positive ions which may be used for the formation of the ion beam, and free electrons which, when suitably accelerated, are capable of ionizing neutral atoms to produce other ions and free electrons. This process is maintained by a continuous supply of neutral atoms, as replacements for the extracted ions, together with electrical energy for the acceleration of the free electrons; the electrical energy is supplied by various techniques, the most common of which are continuous current discharge and radiofrequency or microwave discharge.
Among the most important factors determining the performance of ion generators are the energy yield, in other words the ratio between the energy of the ions in the beam and the energy expended to operate the device, and the mass yield, in other words the ratio between the mass of the ions extracted in the unit of time and the flow of introduced neutral atoms.
The energy yield is adversely affected by the energy required for the maintenance of the plasma in the ionization chamber, since this energy makes only an insignificant contribution to the final energy acquired by the ions in the accelerated beam.
The mass yield is adversely affected by the flow of neutral atoms leaving the device, which is also damaging because, next to the phenomenon of charge exchange in the proximity of the extraction system 2, it is the source of greatest erosion of the extraction system, but is particularly unfavourable to the use of the propellant.
An improvement of the mass yield generally entails a deterioration of the energy yield, since a higher rate of ionization is obtained only at the expense of a greater energy input, but this tends to favour the use of the propellant and therefore the autonomy of the device, which is particularly important in space applications.
In currently known devices, the walls of the ionization chamber consist of a metal, for example steel or molybdenum, or, if the walls have to be dielectric, quartz. The use of a dielectric material is necessary in cases in which the excitation of the plasma in the ionization chamber takes place with a transfer of radio-frequency energy through electrodes or coils external to the ionization chamber. In all the cited cases, the losses of electrons on the walls of the ionization chamber constitute an important factor limiting the performance of the device.
It has now been discovered (and this forms the basis of the invention) that it is possible to improve both of the mentioned yields by modifying the characteristics of the walls of the ionization chamber. In fact, the ions and electrons colliding with the walls may be subject to recombination phenomena, and consequently a cancellation of their electrical charge, with a probability which is particularly high if the walls are electrically conducting, but which is also not insignificant even if these walls consist of dielectric material.